AppendixB SerialPort Information
©NationalInstruments Corporation B-7 SerialHardware and Software for Windows
DTE vs. DCE
In theR S-232 specification,DTE (Data Terminal Equipment) and DCE
(Data Communications Equipment)1refer to the types of equipment on
eitherend ofa serialconnection. In general, DTE and DCE refer to
computerequipment andmodems, respectively. Because the RS-232
specificationmainly involves connecting a DTE directly to a DCE and vice
versa,the pinouts are defined so that cabling is simple. Th at is, a cable
connected a computer to a modem bywirin gpin 1 to pin 1, pin 2 to pin 2,
and so on. This method is known as straight-through cabling.
Figure B-5 showsstraight- through cabling in a DTE-to-DCE interface.
FigureB-5. Straight-Through Cabling in a DTE-to-DCE Interface
Straight-throughcabling is still the standard method to connect a modem to
yourPC. However, because many applications use serialcommunication to
connecttwo or more DTEs without modems, the cabling becomes more
complicated.If two DTEs are wired together using a straight-through cable,
one transmitter is connected to the other transmitter,and one receiver is
connected to theo therreceiver. In this setup, no transmissions can occur.
Thus,t hese applications must use a cabling scheme that connects the
transmitteron one device to the receiver on the other device and vice versa.
This method is knownas null-m odem cabling,because it replaces the two
modems that traditional RS-232 applications would require between the
twoDTEs. To communicate from one serial port to another, use a
null-modem cable.
FigureB -6 showsnull-modem cabling in a DTE-to-DTE interface.
FigureB-6. Null-Modem Cabling in a DTE-to-DTE Interface
1InRevision D of the RS-232 specification, a DCE is a Data Circuit-Terminating Equipment.
RxD
TxD
Pin3
Pin2
DTE
Pin3
Pin2
DCE
Pin3
Pin2
DCE
Pin3
Pin2
DTE
RxD
TxD
RxD
TxD
RxD
TxD
Pin2
Pin3Pin3
Pin2
DTE DTE